1. Field of the Invention
The present invention relates to an elastic wave device and an electronic device using the elastic wave device, such as a portable telephone.
2. Background Art
A conventional elastic wave device is described with reference to the accompanying drawing. FIG. 14 is a sectional schematic view of a conventional elastic wave device.
Conventional elastic wave device 101 has piezoelectric substrate 102, inter-digital transducer (IDT) electrode 103, internal electrodes 104, external electrodes 111, side walls 105, cover 107, insulator 110, and connection electrodes 112.
IDT electrode 103 is disposed on piezoelectric substrate 102. Internal electrodes 104 are disposed on piezoelectric substrate 102 and electrically connected to IDT electrode 103. External electrodes 111 are disposed on insulator 110 and connected to external circuits. Connection electrodes 112 are formed on internal electrodes 104 via electrode base layer 109. Connection electrodes 112 are disposed so as to penetrate through insulator 110, and electrically connect external electrodes 111 and internal electrodes 104 via electrode base layer 109.
Side walls 105 are disposed on internal electrodes 104, on the periphery of IDT electrode 103. Cover 107 is disposed on side walls 105 via adhesive layer 106 so as to cover space 108 above IDT electrode 103.
Further, elastic wave device 101 has cover base layer 113 on cover 107, and cover reinforcing layer 114 on cover base layer 113. This structure reinforces cover 107.
Connection electrodes 112 disposed on the top side of electrode cover layer 109 are formed by two electrolytic plating treatments. Each of connection electrodes 112 has first connection electrode 112a formed by a first electrolytic plating treatment and second connection electrode 112b formed by a second electrolytic plating treatment, on the top side of electrode base layer 109. The horizontal sectional shape of second connection electrode 112b is circular.
In forming second connection electrode 112b, a photosensitive resist is formed on the top surface of first connection electrodes 112a and their peripheries before the second electrolytic plating treatment. Next, a circular mask is disposed on the top side of this resist in the top surface portion of each of first connection electrodes 112a, and the photosensitive resist is irradiated with light from the top surface of the mask. Thereby, the resist on the periphery of the mask is hardened. Next, the device is immersed in a developing solution so that the part of the resist under the mask is dissolved and removed. Thereby, an opening whose horizontal sectional shape is circular is formed. Thereafter, the second electrolytic plating treatment is performed, so that second connection electrode 112b can be formed in this opening.
When the photosensitive resist is irradiated with light from the top surface of the mask, the light passes through the photosensitive resist and radiates to the top surface of first connection electrode 112a. The radiation light is reflected by asperities on the top surface of first connection electrode 112a, and the reflected light radiates to the part of the resist under the mask.
The part of the resist under the circular mask is a circular cylinder whose horizontal sectional shape is circular. The wall surface of the peripheral edge of the circular cylinder serves as a light source that further reflects the light that has been reflected by the asperities on the top surface of first connection electrode 112a. Thus, the light reflected by the wall surface of the circular cylinder is concentrated at the center of the circular horizontal section and forms a converging axis along the vertical axis in the center of the circular cylinder.
For this reason, the photosensitive resist in the vicinity of the converging axis is hardened by the energy of the light concentrated on the converging axis, thus forming a resist block. When the device is immersed in a developing solution thereafter so that the part of the resist under the mask is dissolved and an opening whose horizontal section is circular is formed, the resist block formed in the vicinity of the conversing axis does not dissolve and remains in the opening.
As a result, the reliability of second connection electrode 112b to be formed in the opening by the second electrolytic plating treatment degrades, and thus the elastic wave device has a poor yield (see International Publication No. 2006/106831, for example).